I have 4 females together in my 20 gal. You should be fine putting them together in a 5 as long as you provide ample hiding spots. Unless the one female is unusually aggressive. You might also try putting the less aggressive of the two in first and let her get her bearings for a few days. This way she knows where all the good hiding spots are. The reason they were not doing well together in the 1 gal is that is not even really enough room for 1 betta let alone 2.

__________________
There is a pleasure in the pathless woods,
There is a rapture on the lonely shore,
There is society where none intrudes,
By the deep Sea, and music in its roar:
I love not Man the less, but Nature more,
From these our interviews, in which I steal
From all I may be, or have been before,
To mingle with the Universe, and feel
What I can ne'er express, yet cannot all conceal.
-Lord Byron